Electronic oscillators have long served as local frequency sources for such diverse applications as generating fixed carrier frequencies in radio transmitters and controlling synchronism in digital data processors. With the advent of high frequency, multiple channel data networks, it has become critically important for error free operation to maintain each local oscillator in continuous operation at a single, invariant frequency. In a network providing for transmission of data over narrow bandwidth, radio frequency channels, for example, a minor drift in one oscillator frequency of merely a few cycles will cause interference in neighboring channels resulting in the loss of nearly all data transmitted over the affected channels. Moreover, if the oscillator also serves as a local clock in a data transmission network, the same frequency drift will cause a delay in transmission which will interfere with transmissions by other networks allocated different time slots on the same channel. The effects of frequency drift are compounded if the network depends upon some form of angle modulation for processing transmitted digital data.
Early efforts to avoid the occurrence of frequency drift tended to concentrate on the quality of the oscillator and the stability of its environment with techniques such as using a crystal for controlling the oscillator's frequency and maintaining the crystal inside an oven at a constant temperature. The use of high quality crystals maintained in ovens, however, does not obviate such causes of long term frequency drift as crystal aging. Moreover, high quality crystals mounted inside ovens are not only too expensive for most applications, particularly multi-channel networks where each channel usually has a discrete oscillator, but necessitates a large source of constant power to maintain the oven temperatures. This latter requirement renders high quality, oven-mounted crystals unsuitable for use in remotely deployed networks powered by small batteries.
More recent efforts have concentrated upon development of phase locked loops for detecting and correcting frequency drift of a variable-frequency local oscillator. In one instance, this required the use of a digital counter being enabled by a master clock during a sampling interval to count pulses of the variable local oscillator. At the end of the sampling interval, the contents of the counter are compared by a microprocessor with a predetermined digital word stored in memory. Any difference between the counter contents and the digital word is then used, in successive steps, to vary the frequency of the local oscillator.
Another effort uses programmable microprocessors timed by external frequency standards in phase-locked loops to control the frequencies of a plurality of digitally-variable slave oscillators. The microprocessors compare control words as generated with the same words after those words have been cycled through the phase-locked loops. Then, error signals proportional to differences between the generated and cycled words are applied to the slave oscillators to shift their resonant frequencies.
The precision to which these prior art approaches are able to maintain the frequency of an electronic oscillator is inherently limited by both the duration of their sampling intervals and the bit capacity of their counter registers. Substantially longer sampling intervals are infeasible because longer intervals would limit the availability of the oscillators for their intended purposes while larger registers are expensive and necessarily require microprocessors with larger data handling capacities. Moreover, a satisfactory degree of long-term oscillator stability can be achieved in a phase locked loop approach only by integrating several samplings taken over a large number of oscillator cycles, a procedure which is undesirable because it not only limits the useful availability of an oscillator for other uses but also consumes greater quantities of microprocessor time per oscillator and thereby limits the number of oscillators which can be controlled by a single frequency control system.